Visible light communication system

ABSTRACT

A controller is configured, by decreasing light quantity in only a first slot that is any one of a plurality of slots into which a unit time is divided equally, and increasing light quantity in all the remaining slots except the first slot of the plurality of slots, to assign information to the first slot of the plurality of slots. The controller is furthermore configured to decrease light quantity in second slots that are three or more continuous slots starting from a head of a preamble, which indicates a frame start point, of a frame of an optical-communication signal and to increase light quantity in a slot immediately before the preamble.

TECHNICAL FIELD

The present invention relates to a visible light communication systemthat uses visible light as a communication medium.

BACKGROUND ART

In recent years, a visible light communication system (visible light IDsystem) is put in practice use, which is configured to modulate visiblelight (illumination light) emitted from a luminaire (transmitter), totransmit unique ID information and the like to the luminaire, anddemodulate the ID information with a receiver that receives the visiblelight.

Here, a visible light communication apparatus described in JP 2012-69505A (referred to as “document 1”) is illustrated as a conventional exampleof the transmitter that constitutes the visible light communicationsystem. As showed in FIG. 7, this visible light communication apparatus10 includes a constant current source 1, a smoothing capacitor 2, an LEDsource 3, a load change element 4, a signal generating circuit 6, and aswitch element 5.

The smoothing capacitor 2 is connected between output ends of theconstant current source 1 and is configured to smooth an output of theconstant current source 1. The LED source 3 includes a plurality oflight emitting diodes connected in series between the output ends of theconstant current source 1, and the output of the constant current source1, which is smoothed by the smoothing capacitor 2, is supplied. The loadchange element 4 is for partially changing load characteristic of theLED source 3 by being added to the LED source 3. For example, the loadchange element 4 includes a resistor connected in parallel to a part ofthe plurality of light emitting diodes. The signal generating circuit 6is configured to generate an optical-communication signal of a binary.The switch element 5 includes a switching element (for example, aMOSFET) connected in series with the resistor that constitutes the loadchange element 4, for example. It is switched whether the switch element5 is configured to switch whether or not the load change element 4 isadded to the LED source 3 by switching ON/OFF by theoptical-communications signal of a binary.

That is, the size of a load current that flows through the lightemitting diodes in a time period when the load change element 4 is notadded to the LED source 3 is different from that of a time period whenthe load change element 4 is added to the LED source 3, and therefore,the optical-communication signal can be superimposed on the lightquantity of the visible light communication apparatus 10 and betransmitted.

The optical-communication signal transmitted from the visible lightcommunication apparatus 10 is received by a receiver 20 that includes aphoto IC. The receiver 20 adopts the system of receiving theoptical-communication signal by determining the difference between anoptical output on which the optical-communication signal is notsuperimposed and an optical output on which the optical-communicationsignal is superimposed. By adopting the above system, it is possible todetect modulated light even if the modulated light is small.

Incidentally, a specification (protocol) of the visible lightcommunication is specified to Japan Electronics and InformationTechnology Industries Association standard JEITA CP-1222 (visible lightID system), for example. In this standard, send data is converted to apulse position each unit that includes two bits. Such a conversion(coding) system is called 4 PPM (pulse position modulation). In the 4PPM coding, the signal generating circuit 6 turns on the switch element5 to increase the optical output (light on) when a slot is set to “0”.The signal generating circuit 6 turns off the switch element 5 todecrease the optical output (or light off) when a slot is set to “1”(refer to FIG. 2).

Here, in the above-mentioned standard, a pattern of three slots that areassigned with “1” continuously and nine slots that are assigned with “0”continuously (“111000000000”) is used as a preamble for specifying astarting position of a frame (refer to FIG. 8). The pattern does notoccur even if the 4 PPM coding is adopted to the data. When a fourthslot of the data transmitted immediately before the preamble is assignedwith “1”, four slots assigned with “1” are continuously constituted ofthe fourth slot and the three continuous slots assigned with “1”starting from a head of the preamble. Because the data speed in theabove-mentioned standard is 4.8 kilobits per second, the time period forone slot is 104.2 μs, and the time period for the four slots is 416.7 μsthat is nearly equal to 104.2×4.

Then, because the discharging amount of the electric charge charged inthe smoothing capacitor 2 increases when the continuous off period ofthe switch element 5 is extended to the time period for the four slots,a waveform of the load current may be disturbed largely when a change to“0” from “1” of the last is performed (refer to a time period T in FIG.9). Thus, when the waveform of the load current is disturbed largely,there is a possibility that the optical-communication signal is notmodulated correctly and then a communication error occurs. It is notdesirable that the electrostatic capacitance of the smoothing capacitor2 is enlarged in order to solve such a problem because the dimension ofthe smoothing capacitor 2 enlarges and the component cost of thesmoothing capacitor 2 rises.

SUMMARY OF INVENTION

The present invention has been made in the light of the above-describedproblem, and an object of the present invention is to suppressdisturbance of a waveform of a current that flows through a light sourcein addition to suppression of a cost rise and the like.

A visible light communication system according to an aspect of thepresent invention includes a transmitter and a receiver. The visiblelight communication system is configured to transmit anoptical-communication signal from the transmitter to the receiver withvisible light as a communication medium. The transmitter includes: alight source configured to emit the visible light; a lighting circuitconfigured to supply electric power to the light source to turn on thelight source; and a controller configured to control the lightingcircuit to increase/decrease light quantity of the visible light to beemitted by the light source. The controller is configured, by decreasingthe light quantity in only a first slot that is any one of a pluralityof slots into which a unit time is divided equally, and increasing thelight quantity in all the remaining slots except the first slot of theplurality of slots, to assign information to the first slot of theplurality of slots. The controller is furthermore configured to decreasethe light quantity in second slots that are three or more continuousslots starting from a head of a preamble, which indicates a frame startpoint, of a frame of the optical-communication signal and to increasethe light quantity in a slot immediately before the preamble.

In the visible light communication system, the unit time is preferably atime period of one symbol including two bits in a data part except thepreamble of the frame. The plurality of slots preferably includes fourslots into which the time period of the one symbol is divided equally.The controller is preferably configured to assign two-bit data to thefirst slot of the four slots in which the time period of the one symbolis divided equally. Preferably, the controller is furthermoreconfigured, while the preamble includes slots of which the number isequal to a product of four and an integer of three or more, to decreasethe light quantity in the second slots of which the number is equal tothe integer of three or more, to increase the light quantity in seven ormore slots continuously, and not to set four or more of the second slotscontinuously.

A visible light communication system according to an aspect of thepresent invention includes a transmitter and a receiver. The visiblelight communication system is configured to transmit anoptical-communication signal from the transmitter to the receiver withvisible light as a communication medium. The transmitter includes: alight source configured to emit the visible light; a lighting circuitconfigured to supply electric power to the light source to turn on thelight source; and a controller configured to control the lightingcircuit to increase/decrease light quantity of the visible light to beemitted by the light source. The controller is configured, by decreasingthe light quantity in only a first slot that is any one of a pluralityof slots into which a unit time is divided equally, and increasing thelight quantity in all the remaining slots except the first slot of theplurality of slots, to assign information to the first slot of theplurality of slots. The controller is configured to assign two-bit datato the first slot of four slots into which a time period of one symbolis divided equally, in a data part except a preamble, which indicates aframe start point, of a frame of the optical-communication signal. Thefour slots are the plurality of slots. The one symbol includes two bits.The controller is furthermore configured, while the preamble includesslots of which the number is equal to a product of four and an integerof three or more, to decrease the light quantity in second slots ofwhich the number is equal to the integer of three or more, to increasethe light quantity in seven or more slots continuously, and not to setthree or more of the second slots continuously.

A visible light communication system according to an aspect of thepresent invention includes a transmitter and a receiver. The visiblelight communication system is configured to transmit anoptical-communication signal from the transmitter to the receiver withvisible light as a communication medium. The transmitter includes: alight source configured to emit the visible light; a lighting circuitconfigured to supply electric power to the light source to turn on thelight source; and a controller configured to control the lightingcircuit to increase/decrease light quantity of the visible light to beemitted by the light source. The controller is configured, by decreasingthe light quantity in only a first slot that is any one of a pluralityof slots into which a unit time is divided equally, and increasing thelight quantity in all the remaining slots except the first slot of theplurality of slots, to assign information to the first slot of theplurality of slots. The controller is configured to assign two-bit datato the first slot of four slots except a head slot or a tail slot offive slots into which a time period of one symbol is divided equally, ina data part except a preamble, which indicates a frame start point, of aframe of the optical-communication signal. The five slots are theplurality of slots. The one symbol includes two bits. The controller isfurthermore configured, while the preamble includes slots of which thenumber is equal to a product of five and an integer of two or more, todecrease the light quantity in second slots of which the number is equalto the integer of two or more, to increase the light quantity in eightor more slots continuously, and not to set three or more of the secondslots continuously.

A visible light communication system according to an aspect of thepresent invention includes a transmitter and a receiver. The visiblelight communication system is configured to transmit anoptical-communication signal from the transmitter to the receiver withvisible light as a communication medium. The transmitter includes: alight source configured to emit the visible light; a lighting circuitconfigured to supply electric power to the light source to turn on thelight source; and a controller configured to control the lightingcircuit to increase/decrease light quantity of the visible light to beemitted by the light source. The controller is configured, by decreasingthe light quantity in only a first slot that is any one of a pluralityof slots into which a unit time is divided equally, and increasing thelight quantity in all the remaining slots except the first slot of theplurality of slots, to assign information to the first slot of theplurality of slots. The controller is configured to assign two-bit datato the first slot of four slots except a head slot or a tail slot offive slots into which a time period of one symbol is divided equally, ina data part except a preamble, which indicates a frame start point, of aframe of the optical-communication signal. The five slots are theplurality of slots. The one symbol includes two bits. The controller isfurthermore configured, while the preamble includes slots of which thenumber is equal to a product of five and an integer of two or more, todecrease the light quantity in second slots of which the number is equalto the integer of two or more, to increase the light quantity in eightor more slots continuously, and not set the second slots continuously.

The visible light communication system according to each aspect of thepresent invention has an advantageous effective in that it is possibleto suppress a disturbance of a waveform of a current that flows throughthe light source in addition to suppression of a cost rise and the like.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1D are explanatory views for a preamble of a visible lightcommunication system according to a first embodiment of the presentinvention;

FIG. 2 is an explanatory view for a 4 PPM coding in the visible lightcommunication system according to the first embodiment of the presentinvention;

FIG. 3 is an explanatory view for a preamble of a visible lightcommunication system according to a second embodiment of the presentinvention;

FIG. 4 is an explanatory view for a preamble of a visible lightcommunication system according to a third embodiment of the presentinvention; FIG. 5 is an explanatory view for a 4 PPM-S0 coding of thevisible light communication system according to the third embodiment ofthe present invention;

FIG. 6 is an explanatory view for the preamble of the visiblecommunication system according to the third embodiment of the presentinvention;

FIG. 7 is a block diagram showing a visible light communication system;

FIG. 8 is an explanatory view for a preamble of a conventional example;and

FIG. 9 is an explanatory view for a load current waveform of theconventional example.

DESCRIPTION OF EMBODIMENTS

Hereinafter, a visible light communication system according to eachembodiment of the present invention will be described in detail withreference to drawings.

First Embodiment

As shown in FIG. 7, a visible light communication system according tothe present embodiment includes a transmitter (visible lightcommunication apparatus) 10 and a receiver 20, like the conventionalexample described in the document 1. Since the transmitter 10 of thepresent embodiment has the identical circuit configuration as thevisible light communication apparatus of the conventional example, adetailed description thereof will be omitted. In the transmitter 10 ofthe present embodiment, a constant current source 1 and a smoothingcapacitor 2 correspond to a lighting circuit. An LED source 3corresponds to a light source. A load change element 4, a switch element5, and a signal generating circuit 6 correspond to a controller.

Also in the present embodiment, a 4 PPM coding is performed to databased on Japan Electronic and Information Industrial Associationstandard JEITA CP-1222. That is, the controller (signal generatingcircuit 6) is configured, by decreasing light quantity in only any one(first slot) of four slots into which a unit time (symbol time) isdivided equally, and increasing light quantity in the remaining threeslots, assign information (data) to a position of the above-mentionedslot in which the light quantity is decreased. JEITA CP-1222 specifiesthat “1” is assigned to the slot in which the light quantity isdecreased and “0” is assigned to the slot in which the light quantity isincreased.

Here, as described in Background Art, when a fourth slot of the datatransmitted immediately before the preamble is assigned with “1”, fourslots assigned with “1” are continuously constituted of the fourth slotand three continuous slots assigned with “1” starting from a head of thepreamble. Then, since the discharging amount of the smoothing capacitor2 is increased when the continuous off period of the switch element 5 isextended to a time period corresponding to four slots or more, awaveform of a load current may be disturbed largely when a change to “0”from “1” of the last occurs.

In the present embodiment, the signal generating circuit 6 is configuredto increase the light quantity in the slot immediately before thepreamble. That is, the signal generating circuit 6 is configured totransmit, without changing, the data when the data of one symbol (twobits) transmitted immediately before the preamble is any one of “00”,“01”, and “10” (refer to FIGS. 1A to 1C). The signal generating circuit6 is configured to insert dummy data when the data of the one symbol is“11”. Since the length of the frame (frame length) except the preambleis specified in the standard, even if the dummy data is inserted, thereceiver 20 has no influence in reception of the data by disregardingthe dummy data. The dummy data is preferably any one of “00”, “01”, and“10”.

In the case where the frame is transmitted continuously, the dataimmediately before the preamble depends on CRC (cyclic redundancy check)data of a previous frame, in detail, two-bit data on an MSB (mostsignificant bit) side of the CRC data. Therefore, the value of a payloadof the frame may be adjusted instead of inserting the dummy data so thattwo bits on the MSB side of the CRC data is not set to “11” beforetransmission of the frame. Alternatively, the dummy data may be insertedonly when the two bits on the MSB side of the CRC data are set to “11”.

As mentioned above, in the present embodiment, the controller (signalgenerating circuit 6) is configured, while the preamble includes theslots of which the number (twelve in the illustrated example) is equalto a product of four and an integer of three or more, to decrease thelight quantity in the slots (second slots) of which the number is equalto the integer of three or more (three in the illustrated example), toincrease the light quantity in seven or more slots continuously, and notto decrease the light quantity in four or more slots continuously.Accordingly, disturbance of the waveform of the current that flowsthrough the LED source 3 can be suppressed while suppressing a cost riseand the like.

The visible light communication system according to the presentembodiment described above includes the transmitter 10 and the receiver20. The visible light communication system is configured to transmit theoptical-communication signal from the transmitter 10 to the receiver 20with visible light as a communication medium. The transmitter 10includes: the light source (LED source 3) configured to emit the visiblelight; a lighting circuit (constant current source 1, smoothingcapacitor 2) configured to supply electric power to the light source toturn on the light source; and a controller (load change element 4,switch element 5, signal generating circuit 6) configured to control thelighting circuit to increase/decrease the light quantity of the visiblelight to be emitted by the light source. The controller is configured,by decreasing the light quantity in only the first slot that is any oneof the plurality of slots into which the unit time is divided equally,and increasing the light quantity in all the remaining slots except thefirst slot of the plurality of slots, to assign the information to thefirst slot of the plurality of slots. The controller is furthermoreconfigured to decrease the light quantity in the second slots that arethree or more continuous slots starting from the head of the preamble,which indicates the frame start point, of the frame of theoptical-communication signal and to increase the light quantity in theslot immediately before the preamble.

As with the visible light communication system according to the presentembodiment, the unit time is preferably a time period of the one symbolincluding two bits in the data part except the preamble of the frame.The plurality of slots preferably includes the four slots into which thetime period of the one symbol is divided equally. The controller ispreferably configured to assign the two-bit data to the first slot ofthe four slots in which the time period of the one symbol is dividedequally. Preferably, the controller is furthermore configured, while thepreamble includes slots of which the number is equal to a product offour and an integer of three or more, to decrease the light quantity inthe second slots of which the number is equal to the integer of three ormore, to increase the light quantity in seven or more slotscontinuously, and not to set four or more of the second slotscontinuously.

Second Embodiment

The present embodiment has a characteristic in a composition of apreamble in an optical-communication signal. Compositions of atransmitter 10 and the like are the same as those of the conventionalexample, and then display and description of the detailed compositionsthereof will be omitted.

A controller (signal generating circuit 6) of the present embodiment isconfigured to constitute the preamble with a pattern that fulfills thefollowing four conditions (refer to FIG. 3).

1. The number of slots of the preamble is twelve (the same as the firstembodiment and the conventional example).

2. Only three slots of twelve slots are assigned with “1” (preventionfrom a flicker of illumination light).

3. At least seven continuous slots are assigned with “0” (it does notgenerate in a 4 PPM signal).

4. Three or more slots that are assigned with “1” are not continued.

The “prevention from a flicker of illumination light” that is the objectof the second condition means that the change (flicker) of theluminosity of the illumination light can be prevented by the preamblebeing also constituted at the same ratio as the 4 PPM signal since onlyone slot of the four slots assigned with “1” in the 4 PPM signal.

As mentioned above, in the present embodiment, the controller (signalgenerating circuit 6) is configured, while the preamble includes theslots of which the number (12) is equal to a product of four and aninteger of three or more, to decrease the light quantity in the slots(second slots) of which the number (3) is equal to the integer of threeor more, to increase the light quantity in seven or more slotscontinuously, and not to decrease the light quantity in three or moreslots continuously. Accordingly, disturbance of a waveform of a currentthat flows through the LED source 3 can be suppressed in addition tosuppression of a cost rise and the like. Although being set to 12 (=3×4)in the present embodiment, the number of slots of the preamble may beset to the number of slots that fulfills the first condition like 16(=4×4) and 20 (=5×4).

The visible light communication system according to the presentembodiment described above includes the transmitter 10 and a receiver20. The visible light communication system is configured to transmit anoptical-communication signal from the transmitter 10 to the receiver 20with visible light as a communication medium. The transmitter 10includes: a light source (LED light source 3) configured to emit thevisible light; a lighting circuit (constant current source 1, smoothingcapacitor 2) configured to supply electric power to the light source toturn on the light source; and the controller (load change element 4,switch element 5, signal generating circuit 6) configured to control thelighting circuit to increase/decrease the light quantity of the visiblelight to be emitted by the light source. The controller is configured,by decreasing the light quantity in only a first slot that is any one ofthe plurality of slots into which a unit time is divided equally, andincreasing the light quantity in all the remaining slots except thefirst slot of the plurality of slots, to assign information to the firstslot of the plurality of slots. The controller is configured to assigntwo-bit data to the first slot of the four slots into which a timeperiod of one symbol is divided equally, in the data part except thepreamble, which indicates a frame start point, of a frame of theoptical-communication signal. The four slots are the plurality of slots.The one symbol includes two bits. The controller is furthermoreconfigured, while the preamble includes the slots of which the number isequal to a product of four and an integer of three or more, to decreasethe light quantity in the second slots of which the number is equal tothe integer of three or more, to increase the light quantity in theseven or more slots continuously, and not to set three or more of thesecond slots continuously.

Third Embodiment

The present embodiment has a characteristic in a composition of apreamble in an optical-communication signal. Compositions of atransmitter 10 and the like are the same as those of the conventionalexample, and then display and description of the detailed compositionsthereof will be omitted.

A controller (signal generating circuit 6) of the present embodiment isconfigured to assign two-bit data to a position of one slot, in whichthe light quantity is decreased, of five slots into which a time periodof one symbol is divided equally in a data part except the preamble(refer to FIG. 5). The one symbol includes two bits. Hereinafter, thecoding mode in the present embodiment is called a 4 PPM-S0 coding.

The controller (signal generating circuit 6) of the present embodimentfurthermore constitutes the preamble with a pattern that fulfills thefollowing four conditions (refer to FIG. 4).

1. The number of slots of the preamble is ten (prevention from a flickerof illumination light).

2. Only two slots of ten slots are assigned with “1” (prevention from aflicker of illumination light).

3. At least eight continuous slots are assigned with “0” (it does notgenerate in a 4 PPM-S0 signal).

4. Slots that are assigned with “1” are not continued.

The “prevention from a flicker of illumination light” that is the objectof the first and second conditions means that the change (flicker) ofthe luminosity of the illumination light can be prevented by thepreamble being also constituted at the same ratio as the 4 PPM-SO signalsince only one slot of the five slots is assigned with “1” in the 4PPM-S0 signal.

As mentioned above, in the present embodiment, the controller (signalgenerating circuit 6) is configured, while the preamble includes theslots of which the number (ten in the illustrated example) is equal to aproduct of five and an integer of two or more (two in the illustratedexample), to decrease the light quantity in slots (second slots) ofwhich the number is equal to the integer of two or more (2), to increasethe light quantity in eight or more slots continuously, and not todecrease the light quantity in slots continuously. Accordingly,disturbance of a waveform of a current that flows through an LED source3 can be suppressed in addition to suppression of a cost rise and thelike.

The visible light communication system according to the presentembodiment includes the transmitter 10 and a receiver 20. The visiblelight communication system is configured to transmit anoptical-communication signal from the transmitter 10 to the receiver 20with visible light as a communication medium. The transmitter 10includes: a light source (LED light source 3) configured to emit thevisible light; a lighting circuit (constant current source 1, smoothingcapacitor 2) configured to supply electric power to the light source toturn on the light source; and a controller (load change element 4,switch element 5, signal generating circuit 6) configured to control thelighting circuit to increase/decrease the light quantity of the visiblelight to be emitted by the light source. The controller is configured,by decreasing the light quantity in only a first slot that is any one ofthe plurality of slots into which a unit time is divided equally, andincreasing the light quantity in all the remaining slots except thefirst slot of the plurality of slots, to assign information to the firstslot of the plurality of slots. The controller is configured to assigntwo-bit data to the first slot of the four slots except a head slot or atail slot of the five slots into which a time period of one symbol isdivided equally, in a data part except the preamble, which indicates aframe start point, of a frame of the optical-communication signal. Thefive slots are the plurality of slots. The one symbol includes two bits.The controller is furthermore configured, while the preamble includesthe slots of which the number is equal to a product of five and aninteger of two or more, to decrease the light quantity in second slotsof which the number is equal to the integer of two or more, to increasethe light quantity in the eight or more slots continuously, and not toset three or more of the second slots continuously.

Although the number of slots of the preamble is set to 10 (=2×5) in thepresent embodiment, the number of slots may be set to 15 (=3×5) or thelike as shown in FIG. 6. Even if two slots that are assigned with “1”are continued, the disturbance of the waveform of the current can besuppressed.

The visible light communication system according to the present exampleincludes the transmitter 10 and the receiver 20. The visible lightcommunication system is configured to transmit the optical-communicationsignal from the transmitter 10 to the receiver 20 with the visible lightas the communication medium. The transmitter 10 includes: the lightsource (LED light source 3) configured to emit the visible light; thelighting circuit (constant current source 1, smoothing capacitor 2)configured to supply electric power to the light source to turn on thelight source; and the controller (load change element 4, switch element5, signal generating circuit 6) configured to control the lightingcircuit to increase/decrease the light quantity of the visible light tobe emitted by the light source. The controller is configured, bydecreasing the light quantity in only the first slot that is any one ofthe plurality of slots into which the unit time is divided equally, andincreasing the light quantity in all the remaining slots except thefirst slot of the plurality of slots, to assign the information to thefirst slot of the plurality of slots. The controller is configured toassign two-bit data to the first slot of the four slots except the headslot or the tail slot of the five slots into which the time period ofthe one symbol is divided equally, in the data part except the preamble,which indicates the frame start point, of the frame of theoptical-communication signal. The five slots are the plurality of slots.The one symbol includes two bits. The controller is furthermoreconfigured, while the preamble includes the slots of which the number isequal to a product of five and an integer of two or more, to decreasethe light quantity in the second slots of which the number is equal tothe integer of two or more, to increase the light quantity in the eightor more slots continuously, and not set the second slots continuously.

The invention claimed is:
 1. A visible light communication systemcomprising a transmitter and a receiver, the visible light communicationsystem being configured to transmit an optical-communication signal fromthe transmitter to the receiver with visible light as a communicationmedium, the transmitter comprising: a light source configured to emitthe visible light; a lighting circuit configured to supply electricpower to the light source to turn on the light source; and a controllerconfigured to control the lighting circuit to increase/decrease lightquantity of the visible light to be emitted by the light source, thecontroller configured, by decreasing the light quantity in only a slotthat is any one of a plurality of slots into which a unit time isdivided equally, and increasing the light quantity in all remainingslots of the plurality of slots, to assign information to a position ofthe slot in which the light quantity is decreased, of the plurality ofslots, and the controller furthermore configured to decrease the lightquantity in slots that are three or more continuous slots starting froma head of a preamble, which indicates a frame start point, of a frame ofthe optical-communication signal and to increase the light quantity in aslot immediately before the preamble.
 2. The visible light communicationsystem according to claim 1, wherein the unit time is a time period ofone symbol comprising two bits in a data part except the preamble of theframe, wherein the plurality of slots comprises four slots into whichthe time period of the one symbol is divided equally, wherein thecontroller is configured to assign two-bit data to a position of theslot in which the light quantity is decreased, of the four slots, andwherein the controller is furthermore configured, while the preamblecomprises slots of which a number is equal to a product of four and aninteger of three or more, to decrease the light quantity in the secondslots of which a number is equal to the integer of three or more, toincrease the light quantity in seven or more slots continuously, and notto set four or more of the second slots continuously.
 3. A visible lightcommunication system comprising a transmitter and a receiver, thevisible light communication system being configured to transmit anoptical-communication signal from the transmitter to the receiver withvisible light as a communication medium, the transmitter comprising: alight source configured to emit the visible light; a lighting circuitconfigured to supply electric power to the light source to turn on thelight source; and a controller configured to control the lightingcircuit to increase/decrease light quantity of the visible light to beemitted by the light source, the controller configured, by decreasingthe light quantity in only a slot that is any one of a plurality ofslots into which a unit time is divided equally, and increasing thelight quantity in all remaining slots of the plurality of slots, toassign information to a position of the slot in which the light quantityis decreased, of the plurality of slots, the controller configured toassign two-bit data to the position of the slot of four slots into whicha time period of one symbol is divided equally, in a data part except apreamble, which indicates a frame start point, of a frame of theoptical-communication signal, the four slots being the plurality ofslots, the one symbol comprising two bits, and the controllerfurthermore configured, while the preamble comprises slots of which anumber is equal to a product of four and an integer of three or more, todecrease the light quantity in slots of which a number is equal to theinteger of three or more, to increase the light quantity in seven ormore slots continuously, and not to set three or more of the slots inwhich the light quantity is decreased, continuously.
 4. A visible lightcommunication system comprising a transmitter and a receiver, thevisible light communication system being configured to transmit anoptical-communication signal from the transmitter to the receiver withvisible light as a communication medium, the transmitter comprising: alight source configured to emit the visible light; a lighting circuitconfigured to supply electric power to the light source to turn on thelight source; and a controller configured to control the lightingcircuit to increase/decrease light quantity of the visible light to beemitted by the light source, the controller configured, by decreasingthe light quantity in only a slot that is any one of a plurality ofslots into which a unit time is divided equally, and increasing thelight quantity in all remaining slots of the plurality of slots, toassign information to a position of the slot in which the light quantityis decreased, of the plurality of slots, the controller configured toassign two-bit data to the position of the slot of four slots except ahead slot or a tail slot of five slots into which a time period of onesymbol is divided equally, in a data part except a preamble, whichindicates a frame start point, of a frame of the optical-communicationsignal, the five slots being the plurality of slots, the one symbolcomprising two bits, and the controller furthermore configured, whilethe preamble comprises slots of which a number is equal to a product offive and an integer of two or more, to decrease the light quantity inslots of which a number is equal to the integer of two or more, toincrease the light quantity in eight or more slots continuously, and notto set three or more of the slots in which the light quantity isdecreased, continuously.
 5. A visible light communication systemcomprising a transmitter and a receiver, the visible light communicationsystem being configured to transmit an optical-communication signal fromthe transmitter to the receiver with visible light as a communicationmedium, the transmitter comprising: a light source configured to emitthe visible light; a lighting circuit configured to supply electricpower to the light source to turn on the light source; and a controllerconfigured to control the lighting circuit to increase/decrease lightquantity of the visible light to be emitted by the light source, thecontroller configured, by decreasing the light quantity in only a slotthat is any one of a plurality of slots into which a unit time isdivided equally, and increasing the light quantity in all remainingslots of the plurality of slots, to assign information to a position ofthe slot in which the light quantity is decreased, of the plurality ofslots, the controller configured to assign two-bit data to the positionof the slot of four slots except a head slot or a tail slot of fiveslots into which a time period of one symbol is divided equally, in adata part except a preamble, which indicates a frame start point, of aframe of the optical-communication signal, the five slots being theplurality of slots, the one symbol comprising two bits, and thecontroller furthermore configured, while the preamble comprises slots ofwhich a number is equal to a product of five and an integer of two ormore, to decrease the light quantity in slots of which a number is equalto the integer of two or more, to increase the light quantity in eightor more slots continuously, and not set the slots in which the lightquantity is decreased, continuously.